Recently, the development of electronic paper has been advancing vigorously in business organizations and universities. Contemplated for the future electronic paper application markets are various potential application forms including electronic books heading the list, sub-displays for mobile terminals, display parts for IC cards, etc. One of the most promising modes for the electronic paper is one using a cholesteric liquid crystal. Cholesteric liquid crystals have excellent characteristics such as semi-permanent display maintaining function (memory function), bright color display, high contrast, and high resolution.
Cholesteric liquid crystals are sometimes called chiral nematic liquid crystals. A cholesteric liquid crystal is a liquid crystal in which molecules of nematic liquid crystal form a helical cholesteric phase by adding a comparatively large amount (several tens of percent) of chiral additive (chiral agent). A cholesteric liquid crystal display apparatus performs a display process using orientation states of the liquid crystal molecules.
For the multi-tone display method by cholesteric liquid crystal, various driving methods have been proposed. The driving methods for the multi-tone display by cholesteric liquid crystal are divided into two methods of dynamic driving and conventional driving.
The dynamic driving method requires a complicated control circuit and a driver IC because a driving waveform of the dynamic driving method is complicated. Moreover, a low resistance electrode is required as the transparent electrode of the panel, and this increases the manufacturing cost. Furthermore, the dynamic driving method requires high power consumption. Recently, a dynamic driving method that employs an inexpensive general-purpose driver has been tried. However, there are drawbacks such as the inability to obtain a high contrast display, and there is a trade-off between cost reduction and display quality.
A conventional driving method drives liquid crystal at a comparatively high speed of quasi-video rate so as to gradually change from the planar state to the focal conic state or from the focal conic state to the planar state by using the cumulative time particular to the liquid crystal and adjusting the number of times of application of a short pulse.
When tones are set by using the cumulative time by the conventional driving method, methods are available in which the number of times of application of a short pulse is adjusted and in which the pulse width W is varied. The method in which the pulse width is varied is more advantageous in power consumption suppression than the method in which the number of times of application of a short pulse is adjusted. A method is also available in which the cumulative time of pulse application is varied for both the pulse width and the number of times of pulse application.
Generally, the conventional driving method, compared with dynamic driving method, is more advantageous in that the conventional driving method provides lower power consumption at writing, cost reduction of circuit parts, and more stable high contrast display.
Although the conventional driving method is advantageous in cost and display quality, the method has a drawback in that the writing speed is slower than that of the dynamic driving method. For example, in a display apparatus with a standard panel structure that includes many lines such as XGA specification (1024×768 pixels), for multi-color display of 4096 colors (4 bits for each color, 16 tones), the writing time for one line is approximately 13 ms to 15 ms and the writing time for all lines is approximately 10 to 12 seconds.
In order to achieve faster processing speed, reducing viscosity of liquid crystal is generally known. However, this is not easy to achieve, because there is a trade-off between such reduction of viscosity and electrical and optical properties which are important properties of liquid crystal. Thus, achieving such speed-up by methods other than the method to reduce viscosity of liquid crystal is sought after.